Monster Falls: AU where all the citizens of Gravity Falls are monsters

Relativity Falls: AU where Grunkle Stan & Stanford and Dipper & Mabel's ages are switched; the Stan twins are staying with Grauntie Mabel for the summer. Other older characters become younger (and vice versa) in this AU as well

Reunion Falls: AU where Dipper and Mabel were separated at birth, and meet for the first time during their summer at Gravity Falls

Transcendence AU: A shared universe where Dipper becomes Alcor, a dream demon. Mabel is his human counterpart, Mizar, and is constantly reincarnated into new forms. "The Transcendence" refers to the in-universe event where magic became commonplace.